Serotonin content and melatonin production in the pineal gland of the male Djungarian hamster (Phodopus sungorus)

Diurnal variations in serum melatonin levels and pineal concentrations of serotonin (5-HT), N-acetylserotonin (NAS), 5-hydroxyindole acetic acid (5-HIAA), and melatonin were estimated in adult male Djungarian hamsters kept under long-day (LD 16:8) or short-day (LD 8:16) photoperiods. The nocturnal increase in melatonin production was accompanied by a marked drop in pineal serotonin concentrations. Serotonin levels, however, decreased approximately 4 hr before pineal melatonin increased. Correlations of the mean values for pineal serotonin and pineal melatonin indicated significant correlations at both LD 16:8 (r = -0.92, P less than 0.001) and LD 8:16 (r = -0.85, P less than 0.001). The mean levels of pineal serotonin and serum melatonin were correlated as well (LD 16:8, r = -0.91, P less than 0.001; LD 8:16, r = -0.81, P less than 0.01). The levels of pineal serotonin declined at approximately the same time as serum melatonin levels increased. These data suggest that the drop in pineal serotonin is primarily a consequence of melatonin production (as reflected by increasing serum concentrations). Consequently, pineal concentrations of melatonin may not be the best estimate of actual melatonin production, but a measure of how much melatonin is accumulated within the pineal due to high synthesis rates while the release of the hormone from the gland is limited.